The present invention relates to the use of thin film deposition technology to create high density interconnects on a conventional printed wiring board substrate. More specifically, the present invention pertains to a method for reducing the effects of accumulated stress at the boundaries of conductive and insulative layers. The method of the present invention can be used with or without conventional build-up layers (e.g., a planarized dielectric layer deposited over the upper surface of a completed printed wiring board substrate, typically by the substrate manufacturer, that has a corresponding dielectric layer deposited over the lower surface of the substrate to counterbalance the stress of the upper build-up layer) and is useful for high density integrated circuit packaging of single chip, multi-chip, and support components such as resistors and capacitors. The method of the present invention is also useful for creating interconnections on high density daughter boards that carry packaged devices.
The semiconductor industry continues to produce integrated circuits of increasing complexity and increasing density. The increased complexity of some of these integrated circuits has in turn resulted in an increased number of input/output pads on the circuit chips. At the same time, the increased density of the chips has driven the input/output pad pitch downward. The combination of these two trends has been a significant increase in the connector pin wiring density needed to connect the chips to packages that interface with the outside world and interconnect the chips to other integrated circuit devices.
A number of different technologies have been developed to interconnect multiple integrated circuits and related components. One such technology, based on traditional printed wiring board (PWB) technology that found wide use during the period in which integrated circuits were packaged in surface mount devices like quad flat packs (QFPs), is often referred to as MCM-L or laminate MCM technology. MCM-L technology typically uses copper and insulating dielectric material sub-laminates as building blocks to create the required interconnect structures. The process of forming a copper conductive pattern on the sub-laminate in MCM-L technology typically includes forming a dry film of photo resist over the copper layer, patterning and developing the photo resist to from an appropriate mask and selectively etching away the unwanted copper thereby leaving the desired patterned conductive layer.
Substrates used in MCM-L technology can be manufactured in large area panels providing efficiencies that lower the costs of production. Interconnect solutions using this technology generally have relatively good performance characteristics because of the copper and low dielectric constant (e.g. less than or equal to 4.0) employed. The printed wiring board industry, however, has not kept pace with the advances in semiconductor manufacturing in terms of pad density. As a result, there is a capability gap between semiconductor manufacturers and interconnect printed wiring board manufactures.
In some applications, two or more pieces of laminate are laminated together to form a final structure. Interconnection between the laminated layers can be provided by through hole mechanical drilling, followed by plating. The drilling process is relatively slow and expensive and can require a large amount of board space. As the number of interconnect pads increases, an increased number of signal layers is often used to form the interconnect structure. Because of these limitations, the conventional printed wiring board technology needs to go to a large umber of metal layers (e.g. greater than eight layers) for some of the applications in high density integrated circuit packaging and daughter board fabrication. Utilizing a large number of layers in this context generally increases cost and decreases electrical performance. Also, the pad size limits the wiring density on any given layer with this technology. Thus, MCM-L technology, while useful for some applications, is not capable of providing the connection density required in other applications.
To improve the interconnect density of MCM-L technology, an advanced printed wiring board technology approach called build-up multi-layer has been developed. In this technology a traditional printed wiring board core is the starting point. Standard drilling and plating techniques are used to form plated through holes in the core. From the basic core this conventional build-up approach has many variations. Typically a dielectric layer approximately 50 microns thick is laminated to both the top and bottom major surfaces of the conventionally fabricated printed wiring board substrate. Vias are made in the build-up layer by laser ablation, photo mask/plasma etch, or other known methods. An electroless seeding step is then done prior to a panel plating step that metalizes both the upper and lower surfaces. Subsequent masking and wet etching steps then define a desired conductive pattern over the laminated dielectric layers.
This technology offers a large improvement in terms of density over MCM-L technology without build-up layers; however, such build-up boards require multiple layers in order to meet the developing high density packaging and daughter board requirements. Thus this technology still has limitations.
Another conventional approach used to package high density input/output uses thick film (screen printing) over cofired ceramic substrates. This technology is sometimes referred to as MCM-C, cofired ceramic MCM and thick film MCM technology. Basically, MCM-C technology involves rolling a ceramic mix into sheets, drying the sheets, punching vias, screening the rolled sheets with a metal paste representing the trace pattern on the surface of the ceramic, stacking and laminating all the layers together, then cofiring at a high temperature (e.g. greater than 850 degrees C) to achieve the desired interconnections.
MCM-C construction has found extensive use in high density and high reliability products where the robustness of the high density interconnect package outweighs the cost considerations. The ability to create a hermetic seal in the ceramic improves the ability to withstand environments not tolerable to conventional printed wiring board technology. While this technology is capable of high density packaging applications (e.g. greater than 1000 pads), it is also very costly. Additionally, performance characteristics, such as signal propagation time, are impacted due to the relatively high dielectric constant (e.g. between 5.0 and 9.0) of the ceramic material. MCM-C technology provides higher connection density than MCM-L technology, but is not capable of providing the connection density required for some of today's high density interconnect applications.
A third approach which the high density interconnect and packaging industry has moved toward to address these high density interconnect applications uses thin film MCM technology and is sometimes referred to as MCM-D or MCM deposition technology. Such MCM-D technology includes forming and patterning thin film conductive traces over a common circuit base.
In some applications, MCM-D technology utilizes a low cost, large surface area printed wiring board structure, with or without the use of conventional build-up multi-layers on the printed wiring board, as the common base and as a starting point to meet the high density and low cost interconnect requirements. Such large substrates may have a surface area of 40 cm by 40 cm or more, thereby providing efficiencies that lower the costs of production. This combination of existing conventional high volume printed wiring board technology and advanced thin film deposition technology represents a significant economic advantage and density improvement as compared to the previously discussed MCM-L and MCM-C technologies.
One significant feature of MCM-D technology is that it creates a high interconnect density substrate using thin film processes on only one side of the common circuit base. The high density interconnects are formed by depositing alternating conducting and insulating thin film layers. The total thickness of several of these deposited layers is less than the thickness of a single conventional build-up layer. This eliminates the need for balancing the build-up layers on both top and bottom to prevent warpage of the substrate.
The MCM-D process involves first laying down a layer of an insulating dielectric on the top surface of a common circuit base, depositing a conductive material over the dielectric layer, creating a circuit pattern in the conductive material, then depositing the next insulating and conductive layers. The various layers so created are connected through vias constructed using a variety of known techniques such as wet chemical etch, photo expose and develop or laser ablation. In this way a three dimensional deposited laminated structure is achieved enabling high density interconnect patterns to be fabricated in small physical areas.
Despite the definite advantages of MCM-D technology, there are potential problems that may result in failure modes and performance limitations if the thin film formation process is not properly implemented. One important aspect of the implementation of deposited thin film layers on the surface of printed wiring board substrates is the control of mechanical stresses generated by both processing and operation. Key to control of the these stresses is recognizing the differences in thermal characteristics of the materials in both the substrate and the thin film build-up layers and then to provide a fabrication method and high density deposited build-up layer structure capable of tolerating the mechanical stresses.
The differences in the coefficients of thermal expansion (CTE) between the dielectric materials of choice (e.g., a CTE of between about 50 to 70 parts per million) and metal materials of choice (e.g., a CTE between about 16 to 17 parts per million) in some MCM-D applications generate stresses that can be the source of cracking and failure. While the stresses cannot altogether be eliminated, it is important, through proper design of the thin film structure, to control the stresses to eliminate or at least minimize any adverse affects that could otherwise be created by such stresses.